1. Field of the Invention
The present invention relates to a process for producing an active composite consisting of a support and of an active agent. The support is porous and chemically inert, so as to allow interactions between the active agent, dispersed in the support, and a gaseous or liquid flow. The present invention also relates to such an active composite.
This active composite is designed to be employed exclusively in thermochemical systems.
2. Description of Related Art
In the field of chemical heat pumps based on the thermodynamics of the reaction between a solid and a gas, or of the adsorption of a gas on a solid, use is made of a mixture of a divided material such as expanded graphite and of a solid reactant, for example a salt, or, respectively, of an adsorbent such as a zeolite. The mixture of expanded graphite and of this solid, which is the site of a chemical reaction or of a physical adsorption, exhibits numerous advantages during a chemical reaction or a physical adsorption between the solid and a gas. The expanded graphite, which is in the form of flakelets, has a very large specific surface and allows the gas to diffuse even in a confined environment. In addition, the heat conductivity of the mixture is high.
In document WO 91/15292 it has been proposed to produce an active composite by preparing a support made up of a block of recompressed expanded graphite which is then impregnated from the outside, for example, with a solution of a salt, the impregnated support being subsequently dried to produce the active composite. The active composite thus made has a heat conductivity which is markedly higher than that of the expanded graphite in flakelet form, while retaining a high porosity to gas flows.
Despite undeniable advantages, the active composite prepared by the process described in document WO 91/15292 is not entirely satisfactory. In fact, when the active composite is employed for containing a salt and forming the site of a chemical reaction with a gas, the absorption of the gas into the salt causes an expansion of the volume of the salt crystals, resulting in a deformation of the graphite flakelets around the crystals. In addition, during a desorption of the gas the salt crystals contract. The physical attraction between the crystals and the graphite flakelets causes a deformation of these flakelets when the crystals contract.
This type of active composite finds a use in thermochemical systems in which a block of the composite is arranged in a metal vessel, the wall of the block being in contact with an exchanger, this wall being external to the block in contact with an exchanger-reactor or, on the contrary, internal in contact with exchanger tubes passing through the block. In the gas desorption phase the contraction of the crystals and the deformation of the flakelets give rise to a general contraction of the block, as a result of which the wall of the block becomes detached from the interior of the vessel. This greatly reduces the coefficient of heat transfer between the block and the vessel.
Document FR-A-2626496 describes a process for improving the characteristics of absorption and desorption of a gas by a reaction medium in which the reaction mixture, consisting of a salt, is mixed with expanded graphite and a second expanded product of rigid structure and high penetrability. This second product includes an exfoliated hydrated lamellar mineral, for example vermiculite or perlite.
The mixture described in document FR-A-2626496 is in the form of a low-density powder and contains the salt in pulverulent form. In this document it is not envisaged to recompress the pulverulent mixture in order to form a block of reactant such as that described in document WO 91/15292. However, if an attempt were to be made to form such a block from the pulverulent mixture of document FR-A-2626496, this block would suffer the disadvantages described above, because the salt particles in this mixture, being dispersed both between the flakelets of expanded graphite and between the lamallae of vermiculite, would give rise to the same deformation of the structure during the adsorption and desorption of the gas, as a result of the presence of salt particles in the graphite flakelets.